As federal, state and local requirements for water purity become more stringent, and as the level of environmental concerns mounts ever higher in general, means of purifying water that reaches ground, rivers and lakes to ever higher levels continue to be sought.
The manufacture of aluminum sulfate (alum) solutions is not exempt from these concerns. A present process for manufacturing aluminum sulfate or other aluminum-bearing ores is to digest bauxite ores containing hydrated alumina (Al.sub.2 O.sub.3 H.sub.2 O) with sulfuric acid. The maximum commercial concentration of the aluminum sulfate (alum) produced is about 48.5% by weight.
Even with careful water washing of the bauxite residue however, some of the alum created during manufacture is entrained with some of the wash water on the residue, which residue generally is transferred to containment ponds. This water will then percolate into the soil, carrying with it aluminum and sulfate ions in measurable amounts, unless they are removed from the wash waters first. In order to meet, and preferably to exceed, the minimum levels set by various federal and state standards for drinking water prior to disposal of such water into the public waterways, these ions must be removed from the entrained water.
The present federal drinking water standard requires a maximum of 250 parts per million (hereinafter ppm) of sulfate ion (SO.sub.4.sup.=) and 0.3 ppm of iron. States such as Florida mandate that the level of aluminum ion (Al.sup.+3) be no more than 1.48 ppm.
Further, in order to improve the overall economies of this process, the aluminum in the waste water should be recoverable in a form so that it can be re-used in the aluminum sulfate manufacturing process.
The above-described present process for aluminum sulfate manufacturing produces waste water solutions that contain about 56 ppm of iron, 18,400 ppm of sulfate and 2900 ppm of aluminum, and thus these solutions must be purified before they can be discharged into the public waters. Further, the pH of this by-product solution is only about 3.3, whereas drinking water standards require that water have a pH of between 6.5-8.5. Thus the waste water solution must also be treated, as with an alkali, to increase the pH to within the designated range.
At present the waste water solutions are fed to mud ponds where the particulates and certain ions are removed as the water seeps into the ground water. However, such a process is not controllable and ions such as aluminum are lost and cannot be recovered from such mud ponds.
It is known that aluminum and sulfate can be removed from waste water by treatment with barium hydroxide (Ba(OH).sub.2). As the pH of water increases, the aluminum precipitates out as aluminum hydroxide. The result is that when the water meets the pH standards, the aluminum and sulfate have been removed from the water sufficiently to meet present-day drinking water standards. The resultant hydroxide solids can be recovered by filtration. However, in order to recover the precipitated aluminum from the solids, extensive processing must be done, which is expensive and adds to the cost of the process.
Aluminum and sulfate can also be removed from waste water by treatment with lime (Ca(OH).sub.2). However, this produces an aqueous solution having a high concentration of sulfate (1750 ppm) and up to 2000 ppm of calcium, in addition to aluminum. This level of calcium is believed to be too high for re-use of this solution in aluminum sulfate manufacturing, and thus the calcium must be removed, which is both difficult and very expensive.
The great disadvantage of both of these latter known processes is that reagents are added to the waste water solution, which reagents in turn must be removed before the water can be allowed to be discharged. Further, the aluminum is in a form that is difficult and expensive to re-use in the aluminum sulfate manufacturing process. Thus the search for an economic process that produces high quality water from aluminum-contaminated waste water, which can be discharged into public waterways without further treatment, and that recovers aluminum in a form so that it can readily be re-cycled to the manufacturing process, has continued.